It has been known to detect a rotation of a spur gear (rotor) as a detection target by using a bias magnet for generating a bias magnetic field applied to the spur gear, a first magnetic element pattern constructed with two magnetic elements connected in series and having resistances changing with a change in the bias magnetic field, and a second magnetic element pattern constructed with two magnetic elements connected in series and having resistances changing with a change in the bias magnetic field (for example, refer to Patent Document 1).
Specifically, in the first magnetic element pattern, a connection point between the two magnetic elements forms an output terminal for outputting an output signal. In the second magnetic element pattern, a connection point between the two magnetic elements forms an output terminal for outputting an output signal.
When the spur gear rotates, a magnetic line (magnetic vector) generated from the bias magnet is displaced in the rotation direction. Accordingly, the output signals of the first and second magnetic element patterns change. Then, a comparator compares the output signal of the first magnetic element pattern with the output signal of the second magnetic element pattern and outputs a detection signal indicative of a rotational angle of the spur gear.
In the Patent Document 1, the rotational angle of the spur gear can be detected by comparing the output signal of the first magnetic element pattern with the output signal of the second magnetic element pattern by using the comparator. However, as the distance between the spur gear and the bias magnet increases, the amount of displacement of the magnetic line in the rotation direction (i.e., the amount of change in the angle) decreases. Accordingly, the amount of change in the output signal of each of the first and second magnetic element patterns decreases. Therefore, it is difficult for the comparator to compare the output signals of the first and second magnetic element patterns.
The above problem can occur not only in an apparatus for detecting a rotation of a rotating spur gear but also in an apparatus for detecting a displacement of a detection target.